Compared with vulcanization of rubbers with sulfur, the technique of vulcanization with peroxides used as crosslinking agents offers superior heat resistance, compression set, and electrical insulation for molded articles and superior properties in compounding with colored substances, e.g., coloration and stability of color. Vulcanization with peroxides has therefore enjoyed extensive commercial use in applications where the aforementioned properties are particularly important.
However, as is well known, vulcanization with peroxides is hindered by oxygen and, if the vulcanization is carried out in the presence of air, crosslinking reaction does not proceed to an adequate extent on the surface of the resulting molded article which is in contact with air, leading to various problems such as causing extreme tackiness, reduction in strength, and insufficient hardness. These problems cause defects that the surface of the molded article is readily scratched and is poor in wear resistance and slipperiness.
Accordingly, vulcanization with peroxides is generally applied to the production of rubber products using a vulcanization step shielded from air, such as press molding, injection molding, or transfer molding. In the case of autoclave cure which is a commercialized technique for fabricating hoses and electric wires, products satisfying practical requirements can be obtained by replacing air (oxygen) with steam. However, in order to yield products of consistent quality, utmost care must be exercized in process control.
Continuous vulcanization of rubbers has recently gained commercial acceptance, and industrially important rubber products such as hoses, weather strips, and roofing sheets are currently produced by the continuous vulcanization method. While continuous vulcanization can be performed by various methods, the most commonly employed techniques are hot-air vulcanization (HAV), ultra-high frequency vulcanization (UHF), and fluid bed vulcanization (FBV) process. However, any of these methods perform vulcanization in the presence of air and, hence, are not suitable for vulcanization with peroxides.
The present inventors therefore made extensive investigations to develop a continuous method of vulcanization with peroxides which is capable of providing rubber products with excellent heat resistance and compression set.